1. Technical Field of the Invention
The present invention relates to a photovoltaic module that brings improved reliability.
2. Description of the Related Art
A photovoltaic system directly converts the solar light, which is clean and inexhaustibly supplied, into electricity. For this reason, the photovoltaic system is expected as a new energy source.
Here, in a case of a solar cell constituting the photovoltaic system, an output power per unit is in the order of several watts. For this reason, when a photovoltaic system is used as an electric power source for a house, a building or the like, a photovoltaic module including a plurality of solar cells that are electrically connected to one another in series or in parallel is used. With this module, the output power of the photovoltaic system can be increased up to the order of several hundred watts.
To be more precise, as shown in FIG. 1, a photovoltaic module 100 includes a solar cell 101, a light receiving surface side supporting member 102, a back surface side supporting member 103 and a sealing material 104. The light receiving surface side supporting member 102 is provided along a light receiving surface of the solar cell 101. The back surface side supporting member 103 is provided along a back surface of the solar cell 101. The sealing material 104 seals the solar cell 101 between the light receiving surface side supporting member 102 and the back surface side supporting member 103.
In such a photovoltaic module 100, it is generally desirable to maintain a strong adhesion between the sealing material 104, and both of the light receiving surface and the back surface of the solar cell 101 in order to enhance weather resistance and durability. Here, a technique has been proposed for adding a silane coupling agent to the sealing material 104 in order to mitigate a time deterioration in the adhesive property of the sealing material 104 to the solar cell 101 (for example, Japanese Patent Application Laid-open Publication No. 2000-183382).
When the photovoltaic module 100 does not receives the solar light, a stress a on the light receiving surface side of the solar cell 101 is substantially equal to a stress b on the back surface side of the solar cell 101, as shown in FIG. 1.
In contrast, when the photovoltaic module 100 receives the solar light, the temperature on the light receiving surface side of the solar cell 101 becomes higher than the temperature on the back surface side of the solar cell 101. With this phenomenon, the sealing material 104 is thermally expanded more on the light receiving surface side of the solar cell 101 than on the back surface side of the solar cell 101. Since a coefficient of thermal expansion of the sealing material 104 is greater than that of the solar cell 101, the stress a on the light receiving surface side of the solar cell 101 becomes smaller than the stress b on the back surface side thereof, as shown in FIG. 2.
In this way, when the solar cell 101 receives the solar light, the balance between the stress a and the stress b is disrupted. As a result, the solar cell 101 is warped as shown in FIG. 2.
Here, as shown in FIG. 3, in order to collect photogenerated carriers generated in a photovoltaic body 105, the solar cell 101 is provided with a collector electrode 106 on the light receiving surface of the photovoltaic body 105, and a collector electrode 107 on the back surface of the photovoltaic body 105. Since the collector electrode 106 is provided on the light receiving surface of the photovoltaic body 105, it is preferable to form the collector electrode 106 as narrow as possible in order not to block the reception of the solar light.
However, when the solar cell 101 is warped due to the reception of the solar light, a stress is applied to the collector electrode 106. In addition, since the photovoltaic module 100 is used in the open air, and repeats receiving and not receiving the solar light, damage in the collector electrode 106 is accumulated. For this reason, the electron collection performance of the collector electrode 106 is likely to be lowered. Moreover, the thinner the thickness of a substrate constituting the photovoltaic body 105 is, the more heavily the solar cell 101 is warped. Accordingly, heretofore, the thickness of the substrate cannot be made extremely thin.